International projectionist (Jan-Dec 1936)

SOME HAZARDOUS PROPERTIES OF MOTION PICTURE FILM By A. H. NUCKOLLS AND A. F. MATSON UNDERWRITERS' LABORATORIES, CHICAGO MANY cellulose nitrate film fires and explosions in the past involving loss of life and damage to property have been thoroughly investigated by underwriters and others in order to devise measures to control this hazard. Through the combined efforts of the National Board of Fire Underwriters, the National Fire Protection Association, and manufacturers of cellulose nitrate, regulations governing the handling and storage of inflammable film have been -worked out and have been in use for many years. This paper will discuss some of the hazardous properties of cellulose nitrate film and also the cellulose •acetate (slow-burning type) film. In handling and storing nitrocellulose, •or, chemically speaking, cellulose nitrate, film, the danger is due to the unusually low temperature of ignition or decomposition of the film, its extremely rapid rate of combustion, and the fact that it <;an decompose (exothermic), even in a Testricted supply of air or oxygen, with the evolution of explosive and poisonous gases. Many combustible substances with which we are familiar, such as wood and paper, when heated rapidly do not ignite until a temperature in the neighhorhood of 600 or 700°F. is reached. Cellulose nitrate decomposes when exposed to temperatures in the neighborhood of 300°F. On prolonged exposure decomposition of nitrate film may occur at temperatures as low as 230 °F. Some Common Danger Points The temperature of ordinary incandescent lights, steam pipes, as well as that of lighted cigarettes and matches, exceeds 300°F., and such sources of heat may therefore ignite the film. Hence, in handling and storing cellulose nitrate film, it is of great importance not to have any portion of the film near steam pipes, incandescent lights, or any source •of heat. Everyone is familiar with the fact that the chemical reaction known as combustion is accompanied by the evolution of heat. The amount of heat developed by the combustion of a unit mass of a given substance is known as the heat of combustion. The heat of combustion of a The introduction of the 2000-foot reel as an industry standard has focussed attention upon the hazards incident to the handling of cellulose nitrate film. For their own protection, if nothing else, projectionists should understand thoroughly the dangers inherent in their daily work. The hazards in handling or storing cellulose nitrate motion picture film are due to the unusually low temperature of ignition of the film, the extremely rapid rate of combustion, and the possibility of its decomposing (exothermic) even in a restricted supply of air (oxygen), with the evolution of explosive and poisonous gases. The appended paper, originally presented before the S.M.P.E.,f gives data in respect to the hazardous properties of both cellulose nitrate and cellulose acetate (so-called slow-burning film), with which every projectionist should be familiar. — Editor. given substance is always the same if the conditions of the chemical reaction are the same, regardless of whether the combustion takes place slowly, or rapidly, or occurs in air or in pure oxygen. The difference of temperature that is noticed between the slow and the rapid combustion of substances depends primarily upon the length of time during which the heat of combustion is evolved. In the absence of other factors affecting the result, we may regard the temperature as depending upon the quantity of heat evolved in unit time. The heat of combustion of nitrocellulose is about the same as that of wood, being six to eight thousand BTU's per pound; but the rate of combustion of nitrocellulose is from 12 to 18 times that of wood in the same form. It will be evident, therefore, that the temperature attained by the combustion of cellulose nitrate is extremely high as compared with that of other substances commonly met in practice. Film fires are therefore very difficult to control, and may cause considerable damage within a very short time. Most of the products with which we are familiar are formed from their elements or elementary substances with evolution of heat, and therefore during fjournal of the S.M.P.E. \9i6) No. 6, p. Cu". Vol. XXVII (Dec, decomposition absorb heat. Cellulose nitrate is formed from elementary substances with absorption of heat, therefore upon decomposing into simpler compounds evolves heat so that the process after once starting tends to maintain and accelerate itself. Under practical conditions the decomposition of cellulose nitrate may or may not be accompanied by combustion, depending upon the conditions, particularly as to the air supply. When stored in vaults and other closed places where the supply of air is restricted, decomposition of large quantities of film may occur within a relatively short time. When cellulose nitrate film burns freely in an excess of air the gases evolved are carbon dioxide, nitrogen, and water vapor, none of which is poisonous. Gases Evolved by Nitrate When cellulose nitrate film burns or decomposes (with or without production of flame) in a restricted supply of air, as would be the case in a closet or vault, carbon monoxide (CO), nitrogen dioxide (N02), and nitrogen tetroxide (N204) are evolved. Other gases such as hydrogen, methane, and traces or negligible amounts of hydrocyanic acid gas — and, in the case of undeveloped film, traces of hydrobromic acid — are also evolved. Under ordinary room conditions 1 pound of cellulose nitrate film yields about 4 to 5 cu. ft. of these gases, which, of course, expand as the temperature rises, according to the well known gas law. The proportions of carbon monoxide, oxides of nitrogen, and hydrogen evolved depend in a measure upon the conditions, but the poisonous oxides of nitrogen and carbon monoxide are always produced by the decomposition of nitrate film in dangerous quantity under conditions likely to be met in practice. When the decomposition of the film takes place under pressure, as may be the case in a closed vessel or vault not provided with vents, hydrogen gas is evolved in considerable quantity. It will be noted that when carbon monoxide or when hydrogen gas is evolved under conditions where the supply of air is not sufficient to cause their immediate (Continued on page 25) [23]